Root-form, endosseous dental implants have been in common use in the United States for the treatment of edentulism and partial edentulism since the early 1980's. A root-form, endosseous, dental implant is a cylindrical device made of titanium or titanium alloy, typically 8-15 mm long and 4-6 mm wide. Its surface configuration may be threaded, acid etched, titanium plasma sprayed, or coated with hydroxyappatite. In general, when patients are missing some or all of their teeth, dental implants can be surgically placed into the jaw bone in the areas of the missing teeth. Following a 3-6 month healing period, a second surgical procedure is performed. During the second surgery, the dental implants are exposed and a cylindrical extension device, i.e., an abutment, is screw fastened to each dental implant. When the surgical wound is closed following this surgery, the abutments project through the gingival tissues so that approximately 1-3 mm of the abutment is apparent, extending into the oral cavity proper. At this point in the dental implant therapy, construction of a prosthesis is initiated. The final prosthesis is designed to directly or indirectly attach to the dental implants.
When a patient is missing several, but not all of his teeth, i.e., is partially edentulous, two or more dental implants may be surgically placed to support partial denture prostheses, i.e., "dental bridges". Typically in this scenario, the dental implants are surgically placed a minimum of approximately 3-5 mm apart in the area of the patient's jaw in which teeth are missing. Following healing, as previously described, the most common partial denture prosthesis involves the construction of a metal ceramic restoration. A metal substructure or framework is fabricated in an appropriate dental casting alloy. This metal framework may secondarily be veneered with a dental porcelain to create the illusion of natural dentition. This partial denture prosthesis is then fastened to the dental implants using a series of metal retaining screws, tightened to predetermined torque specifications. The implant supported partial denture prosthesis restores comfortable and efficient oral function, as well as providing acceptable aesthetic replacement of the patient's missing teeth.
Frequently, when a patient is missing all of his teeth, i.e., is edentalous, three to five implants will be surgically placed approximately 10-15 mm apart in the anterior aspect of the patient's edentulous jaw. Following healing, as previously described, a single metal framework or bar can be fabricated and fastened to each of the implants by small metal screws. A complete denture prosthesis is then constructed so that it may be clipped onto the metal bar at the patient's discretion. The metal bar screw fastened to the dental implants serves to stabilize, support and retain the complete denture. This mechanical advantage allows the patient to wear the complete denture prosthesis comfortably with assurance that the denture will not "fall out" at an inopportune and embarrassing moment. This physical and psychological security is frequently not available for patients that do not have the advantage of dental implant support for their complete denture. To this end, dental implants have served to improve a patient's quality of life.
Clinical research supports the ability of the human jaw bone to biologically accept implanted metallic structures, and maintain these structures in a healthy state. It is also the current belief of the dental profession that the application of excessive mechanical force to dental implants may adversely affect the surrounding bone. Excessive force may originate from a variety of sources. One such source is the constant mechanical force applied to implants when an inaccurately fitting metal bar or framework is screw fastened to the implants. This constant mechanical load on the system has been referred to as static prestress. If the fastened bar or framework does not fit accurately onto the dental implants, it may prestress or preload the system leading to (1) biological breakdown of bone contacting and surrounding the dental implants, and/or (2) premature mechanical failure of the retaining screws, dental implants or the metal bar or framework.
The present invention, as described herein, is equally applicable to the construction of metal bars and metal frameworks used in the dental prosthetic rehabilitation of patients missing some or all of their teeth. For simplicity of discussion, the metal bar, as described above in the restoration of edentulous patients, will be discussed in detail. However, it must be emphasized that the present invention may be applied equally well to both the metal bar and the metal framework supported by dental implants.
Bar fabrication involves the "lost wax" technique of metal casting. A wax or resin pattern of the proposed bar is created, incorporated in a gypsum-based investment material and cast in dental alloy, e.g., ADA type IV gold casting alloy consisting of 59% Au+23% Ag+13% Cu+4% Pd. Alternatively, the dental alloy may be nickel-chromium alloy or titanium alloy. Upon completion, the bar is properly finished and polished and placed in the patient's mouth. A single retaining screw is fastened to hold the bar in place, and the other bar-implant interfaces or junctions are visually inspected to assess the adequacy of bar adaptation. This approach of assessing fit may be referred to as the "single screw test". The accuracy of fit of the metal bar to the dental implants can be affected by a variety of factors during bar construction. However, a primary concern is the impact of solidification shrinkage of the cast metal alloy on the resultant three-dimensional accuracy of the bar. It is possible that distortion of the bar may be expressed in all three linear and three rotational dimensions.
As contemplated by the prior art, if adequate fit of the metal bar to the dental implants is not present, procedures directed at correction of the bar must be undertaken. Typically, the metal bar is sectioned, reoriented in the mouth or on the dental cast, and subjected to laboratory brazing procedures. The metal bar must again be placed on the dental implant using the "single screw test" to reassess the accuracy of fit. This sectioning-brazing procedure may have to be performed two, three, or four times before a visually assessed, accurate fit of the metal bar to the dental implants is achieved. A bar containing several soldered or brazed joints may be mechanically weakened such that normal masticatory loading will result in failure.
The present invention is directed to a method and apparatus for constructing accurately fitting, screw-retained, metal bars or metal frameworks for dental implant-assisted prostheses.